In the context of the present disclosure a portable electronic device should be understood as a small microelectronic device designed to be worn on the human body. Especially the devices may be adapted to be at least partly worn at or in the human ear. Examples of such devices include hearing aids and some types of portable electronic sensor systems.
A variety of bearing aid types exist and a few of these are further described below. Behind-The-Ear (BTE) hearing aids are worn behind the ear. To be more precise an electronics unit comprising a housing containing the major electronics parts thereof is worn behind the ear. An earpiece for emitting sound to the hearing aid user is worn in the ear, e.g. in the concha or the ear canal. A connector connects the earpiece to the housing. In a traditional BTE hearing aid, a sound tube is used because the output transducer, which in hearing aid terminology is normally referred to as the receiver, is located in the housing of the electronics unit. In some modern types of hearing aids a conducting member comprising electrical conductors is used, because the receiver is placed in the earpiece in the ear. Such hearing aids are commonly referred to as Receiver-In-The-Ear (RITE) hearing aids. In a specific type of RITE hearing aids the receiver is placed inside the ear canal. This is known as Receiver-In-Canal (RIC) hearing aids.
In-The-Ear (ITE) hearing aids are designed for arrangement in the ear, normally in the funnel-shaped outer part of the ear canal. In a specific type of ITE hearing aids the hearing aid is placed substantially inside the ear canal. This type is known as Completely-In-Canal (CIC) hearing aids. This type of hearing aid requires a very compact design in order to allow it to be arranged in the ear canal, while accommodating the components necessary for operation of the hearing aid.
Other types of hearing aids include cochlear implants and bone conducting hearing aids. Other devices that resemble hearing aids are e.g. devices for the treatment of tinnitus and devices for relieving stress and anxiety.
A great variety of portable electronic sensor systems exist that qualify as portable electronic devices in the context of the present disclosure. One variety is systems comprising means for EEG monitoring. These systems are applicable for a lot of medical purposes such as:
monitoring the users brain waves for evaluation of the result of a medical treatment;
monitoring the user's brain waves for detection of medical states, and possibly alerting the user, caretakers or relatives, wherein examples of such medical states are e.g. impending hypoglycemia and epileptic seizures;
monitoring the user's brain waves for the purpose of diagnosing medical conditions.
Examples of such conditions are epileptic conditions such as absence epilepsy, neurodegenerative conditions such as Parkinson's disease and psychiatric disorders such as Schizophrenia or Anxiety disorders;
providing Audio Feedback for the purpose of treating a disease or a disorder such as Attention Deficit Hyperactivity Disorder (ADHD), tinnitus or phantom pain sensations; and
providing a Brain-Computer Interface or Man-Machine Interface for enabling the user to control the device it-self or for controlling peripheral devices.
Other types of portable electronic devices within the context of the present disclosure further include e.g. cameras, mobile phones and remote controls.
A fuel cell for a portable electronic device is restricted with respect to operating temperature, size, duration of fuel cell operation before re-fuelling is required, magnitude of the output voltage, possible safety issues related to the general fuel cell handling and the range of allowable operating orientations.
A fuel cell for a portable electronic device is capable of functioning at room temperature and encompasses a volume of less than 50 cm3.
A number of fuel cells have the potential to fulfill the above mentioned requirement including e.g. Direct Alcohol Fuel Cells (DAFCs), wherein Direct Methanol Fuel Cells (DMFCs) is a particular attractive type, and Direct Formic Acid Fuel Cells (DFAFCs). Further details concerning DMFCs can be found e.g. in U.S. Pat. No. 5,599,638.
This category of fuel cells generally comprises cells using a polymer electrolyte membrane, also referred to as a proton exchange membrane, where the protons are supplied through a catalytic process of the fuel. In the direct alcohol fuel cell (DAFC) an alcohol is directly oxidized. The most widely used fuel in the DAFC is methanol, thus termed direct methanol fuel cell (DMFC).
Pure methanol and ethanol have 17 and 20 times larger energy density by weight, respectively, than e.g. a type 312 Zn-Air battery. When comparing energy density by volume the numbers are 4 and 5 times, thus, ideally providing at least a 4-fold increase in operating time (not counting the system volume of the fuel cell). Furthermore, the fuel cell will be capable of being recharged in a matter of minutes by simply replenishing the fuel. Finally the energy required to manufacture Zn-Air batteries is typically orders of magnitude larger than the energy required to manufacture e.g. a DMFC.
Rechargeable nickel-metal hydride (NiMH) batteries are also available in typical hearing aid battery sizes. These batteries have a capacity in the range between 10 and 70 mAh, which is only one tenth of the corresponding Zn-air batteries.
The technology of a DAFC can roughly be divided into three main fields; the polymer electrolyte membrane, the catalysts/electrode part and the general system/cell structuring. The latter ensures that the fuel reaches the catalyst layer where it is electrochemically oxidized to form, in the case of alcohols, electrons, protons and carbon dioxide. As the membrane ideally only allows proton conduction (strictly speaking it is H3O+ that is conducted through the membrane), the free electrons are conducted by the electrode layer through an external load and returned to the cathode side. On this side the system structuring allows a flow of air thus providing oxygen, which in term is reduced by the catalyst to form water together with the electrons from the connected load and the protons conducted through the membrane.
Reference is now made to FIG. 6 which illustrates highly schematically a fuel cell for a portable electronic device according to the prior art. The fuel cell 10 comprises a Proton Exchange Membrane (PEM) 11, a negative electrode (anode) 12, a positive electrode (cathode) 13, a fuel reservoir 14, an anode fuel inlet 15, an anode outlet 16 (for removal of the gas produced at the anode), a cathode inlet 17 and a cathode outlet 18 (for removal of the water produced at the cathode). The fuel cell 10 provides current to the external electrical load 20 via the electrical connectors 21a and 21b, which connects the electrical load 20 with the anode 12 and the cathode 13.
Common to all the above mentioned fuel cells is that they generate CO2 or an intermediate. It is a well known issue in any CO2 generating fuel cell that the CO2 needs to be somehow transported out of the fuel cell.
Due to the size limitations imposed on fuel cells for portable electronic devices active components for pumping of fluids in the fuel cells are not an option and the CO2 therefore has to be managed passively.
It is known in the art of fuel cells to provide passive degassing using bubble actuated pumping principles. Such systems are known from e.g. the article “Capillary-driven pumping for passive degassing and fuel supply in direct methanol fuel cells” in Micro fluidics and Nano fluidics, vol. 7, no. 5, 2009 by Paust et al.
However, even such passive systems are not suitable for fuel cells for portable electronic devices due to the size limitations.
Additionally, the use of portable electronic devices is generally characterized by the fact that the orientation of the device, and hence the fuel cell, is not known in advance and may even change during normal operation. This may be a problem since many designs rely on gravity to assist in removing the CO2 bubbles from the Membrane Electrode Assembly (MEA) and to a gas permeable valve. Consequently passive degassing based on a gas permeable exit hole or membrane valve is not well suited for portable electronic devices, because the formed CO2 may tend to remain at, and therefore block, the fuel cell MEA instead of leaving it.
Another issue with systems based on a gas permeable exit hole or membrane valve is that liquid fuel will tend to be pressed out instead of the CO2 if the CO2 does not reach the exit hole.
It is therefore a feature of the present invention to provide a fuel cell for a portable electronic device with improved performance.
It is yet another feature of the present invention to provide a method of manufacturing such fuel cell reservoir.